Nitric oxide is a small diatomic molecule with multiple biological actions, including inhibition of platelet adhesion and aggregation, and relaxation of vascular and non-vascular smooth muscles. Nitric oxide has also been reported to have anti-inflammatory, anti-bacterial and anti-viral properties (see, review by Moncada et al., Pharmacol. Rev., 43:109-142 (1991)). In the gaseous state, nitric oxide exists as a lipophilic molecule in a neutral redox state (NO). Nitric oxide is a complex molecule since it is able to exist in multiple redox states under different physiological conditions. It can also formally exist in charged forms i.e., nitrosonium (NO.sup.+) or nitroxyl (NO.sup.-), or as the neutral species, nitric oxide (NO.multidot.). In biological tissues, nitric oxide has a very short half-life, estimated at less than one second.
One of the potent actions of nitric oxide in mammals is to relax vascular and non-vascular tissue, and, as such, either nitric oxide or an adduct that delivers nitric oxide, is useful as a vasodilator. In the mammalian body, endogenous nitric oxide is produced through an enzymatic reaction in which nitric oxide synthases use L-arginine and molecular oxygen for the synthesis of nitric oxide and citrulline. One of the actions of nitric oxide is believed to be the activation of a soluble form of guanylate cyclase, a cellular enzyme, which catalyzes the formation of 3',5'-cyclic guanosine monophosphate (cGMP). cGMP is believed to act on other cellular targets to mediate the relaxation of vascular smooth muscle and provide the therapeutic effect of vasodilation. Another action of nitric oxide is believed to be the regulation of Na.sup.(+) -K.sup.(+) -ATPase.
The synthesis of nitric oxide from L-arginine by nitric oxide synthase occurs in two steps, each of which requires NADPH. In the first step, an intermediate N-hydroxyguanidine product, N.sup.G -hydroxy-L-arginine, is synthesized by the incorporation of an oxygen into the guanidine function of the L-arginine molecule. In the second step, a second oxygen is incorporated into N.sup.G -hydroxy-L-arginine to form L-citrulline and nitric oxide. (Fukuto et al, in Methods in Nitric Oxide Research, Feelisch et al, eds., John Wiley & Sons, Ltd., pp. 147-160 (1996)). Under an environment of low oxygen tension, however, the synthesis of nitric oxide is greatly reduced. (Furchgott et al, Nature, 288(5789):373-376 (1980); Johns et al, Circ. Res., 65(6):1508-1515 (1989)).
Several clinical conditions are associated with low oxygen tension, such as sexual dysfunctions (Kim et al, J. Clin. Invest. 91(2):437-442 (1993)), pulmonary diseases (including respiratory distress syndrome, asthma, bronchitis/emphysema, and chronic obstructive pulmonary disease) (Howes et al, Thorax, 51(5):516-519, (1996); Fagan et al, Biochem. Biophys. Res. Commun., 254(1):100-103 (1999)), circulatory hypoxia (including heart failure, strokes, and shock), specific organ hypoxia (in which decreased circulation to a specific organ resulting in localized circulatory hypoxia can be due to organic arterial obstruction or can result as a consequence of vasoconstriction, e.g., Raynauds Syndrome) (Agusti et al, Eur. Respir. J., 10(9):1962-1966(1997)), localized hypoxia (which can result from venous obstruction and resultant congestions and reduced arterial blood inflow), edema (which increases the distance through which oxygen diffuses before it reaches the cells can also cause localized hypoxia), central nervous system disorders, memory loss, and arterial disease (Weitzberg et al, Acta. Physiol. Scand., 143(4):451-452 (1991)).
Respiratory distress syndrome, in a child or adult, has severe consequences in the vasculature, such as pulmonary hypertension. Arterial insufficiency of the blood vessel of the penis leads to hypoxic ischemia of this tissue, which limits the synthesis of nitric oxide, and, therefore, limits the erectile capacity.
Increased oxygen requirements can also lead to low oxygen tension. For example, if the oxygen consumption of a tissue is elevated without a corresponding increase in volume flow per unit time, then the oxygen tension (Pa.sub.O2) in the venous blood can be reduced. This can also occur when the hemoglobin is qualitatively and quantitative normal. Examples of such situations include fever and thyrotoxicosis in which cardiac output cannot rise normally, and also in cases in which metabolic rates of oxygen consumption are high.
It would be desirable to increase the production of nitric oxide in tissues under low oxygen conditions to activate the chain of biochemical and cellular events that lead to vasodilation. The present invention is directed to these, as well as other, important ends.